Crosslinked polyrotaxanes are produced by crosslinking polyrotaxanes obtained by introducing capping groups to both ends of a pseudopolyrotaxane. When the pseudopolyrotaxane, for example, is a compound containing a polyethylene glycol (hereinafter also referred to as “PEG”) chain (hereinafter also referred to as “PEG compound”) and a cyclodextrin that includes the PEG compound, the resulting crosslinked polyrotaxane has a structure in which the cyclodextrin threaded onto the linear molecule of the PEG compound in a skewered manner is movable along the linear chain molecule (pulley effect). External force applied to the crosslinked polyrotaxane thus can be uniformly distributed. The crosslinked polyrotaxanes therefore have excellent characteristics, such as resistance to cracks and flaws, that conventional crosslinked polymers lack.
Pseudopolyrotaxanes used for the production of crosslinked polyrotaxanes are obtained in an aqueous dispersion form as they are typically produced by mixing a polyethylene glycol and a cyclodextrin in an aqueous medium.
Patent Literature 1 discloses a method for producing a polyrotaxane, including: dissolving carboxylated polyethylene glycol (hereinafter also referred to as “PEG-BC”) whose PEG chain are carboxylated at both ends and α-cyclodextrin each in 70° C. warm water, mixing the obtained solutions together and cooling the mixture to prepare an aqueous dispersion of pseudopolyrotaxane, freeze-drying the aqueous dispersion to prepare a pseudopolyrotaxane, and reacting the resulting pseudopolyrotaxane with adamantanamine in dimethylformamide in the presence of a BOP reagent (benzotriazole-1-yl-oxy-tris(dimethylamino)phosphonium hexafluorophosphate) as a condensing agent and diisopropylethylamine as a catalyst.
Patent Literature 2 discloses that an aqueous dispersion of pseudopolyrotaxane can be obtained by dissolving a polyethylene glycol and a cyclodextrin into an aqueous medium by heating to 75° C. with stirring, then cooling the mixture to 5° C. with stirring, and continuing stirring at the same temperature for additional 10 hours.
For improved properties of a crosslinked polyrotaxane, it is important that the inclusion amount of the cyclodextrin of the pseudopolyrotaxane is appropriate.
Patent Literature 3, for example, teaches as follows. If the inclusion amount of the cyclodextrin is too small, the resulting crosslinked polyrotaxane has a low crosslinking density, and thus has insufficient dynamic characteristics. If the inclusion amount of the cyclodextrin is too large, that is, if cyclodextrin is too closely packed along the linear molecule, a sliding mode function, in which cyclodextrin molecules relatively move along the linear molecule to maintain a certain distance between them, is not sufficiently exhibited. As a result, the resulting crosslinked polyrotaxane is insufficient in dynamic characteristics such as stretchability or breaking strength.